1. Field of the Invention
The present invention relates to a hydraulic control system for use in a motor vehicle and a method for controlling the hydraulic control system, and, more particularly, to a hydraulic control system in which a solenoid valve that supplies a prescribed fluid pressure and a relay valve that selectively switches the destination of the prescribed fluid pressure are employed and a method for controlling the hydraulic control system.
2. Description of the Related Art
In conventional automatic transmission for a motor vehicle a plurality of gear steps of different speed ratios may be established by selectively engaging a plurality of hydraulic engaging devices. Japanese Patent Application Publication No. 2000-249219A (JP 2000-249219A) describes a hydraulic control system that independently controls the engagement pressures of the hydraulic engaging devices using separate solenoid valves. In addition, Japanese Patent Application Publication No. 2001-248725A (JP 2001-248725A) describes a technique that utilizes a single solenoid valve to concurrently execute an engagement pressure control for a hydraulic friction engaging device of a forward-reverse shifting apparatus and a torque capacity control for a lock-up mechanism in a continuously variable transmission that includes a fluid coupling device provided with a lock-up mechanism. Using the described technique, the destination of a control fluid pressure produced by the solenoid valve is switched by a relay valve (switching valve). Thus, the engagement pressure control, in which the engagement pressure is supplied to the friction engagement device, and the torque capacity control, in which a control fluid pressure is supplied to the lock-up mechanism, may be selectively executed by using the single solenoid valve.
By applying the technique of JP2001-248725A to the automatic transmission of JP2000-249219A, it is possible to provide an automatic transmission in which wherein a solenoid valve that controls the engagement pressure of a specific hydraulic friction engagement device may also execute the torque capacity control of the lock-up mechanism. In this case, however, a failure of an electromagnetic valve that controls switching operation of the relay valve or a failure of the relay valve per se, may result in a switching malfunction in which the relay valve becomes stuck to a flow path through which the control fluid pressure is supplied to the lock-up mechanism to control the torque capacity thereof. When such a switching malfunction occurs, if the control fluid pressure is produced from the solenoid valve to engage a specific hydraulic friction engagement device, the lock-up mechanism may be engaged even when such engagement is not necessary. This makes it necessary to employ a means to detect the switching malfunction of the relay valve. A fluid pressure switch may be used to detect the switching malfunction of the relay valve by detecting the fluid pressure of the solenoid valve produced from the relay valve. With this detection method, however, it is impossible to detect the switching malfunction of the relay valve if no fluid pressure is produced from the solenoid valve. In the event of failure of the solenoid valve per se, it is difficult to reliably distinguish between a non-functional solenoid valve or a relay valve affected by the switching malfunction.